Changes in the number of postsynaptic AMPARs occur during synaptic maturation and during rapid and long term changes in synaptic efficacy. As a consequence, the study of how AMPARs are delivered to, endocytosed from and recycled back to the postsynaptic membrane is a major area of interest in the field of synaptic plasticity. To date, phosphorylation of AMPARs has been studied as a well-established posttranslational modification known to contribute to receptor trafficking to and from the postsynaptic membrane. We have recently discovered ubiquitination as a new posttranslational modification that controls the internalization and endocytic sorting of AMPARs. We have identified the E3 ubiquitin ligase Nedd4-1 as a critical enzyme in AMPAR modification by multiple independent biochemical and functional criteria and we have uncovered a previously unsuspected interplay between receptor phosphorylation and ubiquitination. This proposal builds on these new observations linking ubiquitination and phosphorylation in AMPAR trafficking and synaptic function. We will determine the molecular mechanisms and function of AMPAR ubiquitination in neurons by accomplishing these Specific Aims: 1) to determine how synaptic activity regulates AMPAR ubiquitination and determine the relationship between AMPAR phosphorylation and ubiquitination; 2) to determine how synaptic activity and the interplay between AMPAR phosphorylation and ubiquitination regulates the trafficking and turnover of AMPARs; and 3) to validate and determine the physiological significance of AMPAR ubiquitination on synaptic function. We expect advancements made in this research to provide new insights into the mechanisms of synaptic plasticity and to lay the groundwork for future studies involving AMPAR trafficking and synaptic dysfunction.